N-Heterocyclic carbenes (NHCs) are considered as an important class of compounds which have found application in various fields such as organic synthesis, catalysis and macromolecular chemistry. In the field of catalysis, NHC ligands are often compared with phosphine-based ligands which still remain more commonly used. However, unlike their phosphine analogues, metal-NHC complexes are now recognized for their unique properties and their higher air and moisture stability, particularly in the case of palladium and ruthenium-NHC catalysis.
The importance of NHC ligands in catalysis is related to their σ-donating properties as well as their steric hindrance which has strong effects on both oxidative additions and reductive eliminations in metal-catalyzed organometallic cross-couplings. Spectacular reactivity has been attributed to a “flexible steric bulk” in which the ligands can adjust towards incoming substrates, while enabling the stabilization of low-valent active intermediates.
As an example, the bulky IPr NHC ligand (A, below) reported by Nolan (Ref 1) exhibits excellent properties in catalysis. However the rigidity of the IPr ligand can be envisaged as a significant limitation and the design of new NHC ligands possessing flexible bulk appears as a major challenge.
